Our rsync package was shipped with bundled zlib to provide compatibility with the old-style --compress option up to version 3.1.0. Version 3.1.1 was released on 2014-06-22 and is shipped by all major distributions now.
So we decided to finally drop the bundled library and ship a package with system zlib. This also fixes security issues, actual ones and in future. Go and blame those running old versions if you encounter errors with rsync 3.1.3-3.